Fusion down-draw is a leading technology developed by Corning Incorporated, Corning, N.Y., U.S.A. for making thin, precision glass sheets suitable for use as liquid crystal display (LCD) glass substrates and other opto-electronic devices. This process involves introducing a stream of molten glass into a forming trough called isopipe having two side surfaces converging at a line called root, allowing the glass melt to flow over both top surfaces of the trough of the isopipe called weirs, down along both side surfaces of the isopipe as two molten glass ribbons, join and fuse at the root to form a single glass ribbon, which is then drawn down and cooled below the root to form the glass sheet with desired dimension. In the zone below the root, the glass ribbon travels substantially vertically downward while being drawn and cooled from a viscous state, to visco-elastic and eventually to substantially elastic. The elastic glass ribbon is then cut into individual glass sheets, subjected to further finishing such as edge rounding and polishing, and then packaged and shipped to LCD panel makers for use as TFT or color filter substrates. Cutting of the glass ribbon at below the isopipe typically involves the scoring of the ribbon surface, followed by bending along the score-line, whereby discrete glass sheets are separated from the ribbon and then transferred away.
One of the advantages of the fusion down-draw process for making glass sheets is that the surface quality of the glass sheets is high because the quality areas thereof were only exposed to the atmosphere and never to a solid material such as the isopipe surface or the forming equipment. This process has been used successfully for making glass sheets having a width as large as 3000 mm and a thickness of about 0.6 mm.
The size of LCDs for the consumer electronics market has increased steadily in the past decade, and along with the image quality desired. These have fueled the demand of large-width glass substrates and posed increasingly more stringent requirements for glass sheet quality, such as edge warp and waviness, sheet warp, surface waviness and roughness, thickness uniformity, as well as stress. In addition, consumers have demonstrated interest in lighter electronics, which call for thinner glass substrates having a thickness of 500 μm, 400 μm, 300 μm or even lower.
Making large-size and/or thin glass sheets using the fusion down-draw process is no easy undertaking. Over the years, experts such as the present inventors have gained insights into the many process parameters that can impact the desired attributes of the glass sheets. For example, it was discovered that the separation of glass sheets from the ribbon when the glass is substantially elastic can lead to undesirable motion of the glass ribbon above the separation line, which can propagate into the visco-elastic zone, and even the viscous zone, and thereby affect the attributes of the sheet formed in those zones.
Therefore, there remains a need of an apparatus and method for making glass sheets with desirable attributes at an acceptable yield with improved ribbon stability. The present invention satisfies this and other needs.